1. Field of the Invention
This device refers to a pressure relief valve for tanks carrying liquified gas, and especially for liquified gas tankers.
2. Description of the Prior Art
It is known to provide a pilot valve with an adjusted set pressure to operate a main valve which has an inlet at the tank followed by a valve seat with a lateral gas outlet. A control pressure space in the main valve mechanism is connected for control by the pilot valve such that a movable main valve part, for example a piston, is positioned by the gas pressure from the tank, via the pilot valve. The pilot valve outlet is applied to an inlet side of the control pressure space, which space exerts a closing pressure on the main valve, by pressing the piston towards its valve seat. When the pressure in the tank exceeds the pressure set by the pilot valve, the main valve body is lifted from its seat, and opens the connection between inlet and outlet to vent gas.
Safety relief valves of this type are shown for example by German Patentschrift No. 27 31 804 and by illustrations of AGCO safety relief valves of the series 95. The main valve of this safety relief valve has a piston with an O-ring seat. The tank pressure is conducted by the pilot valve into a control pressure space (also known as "dome" space) on the upper side of the piston. The lower piston surface, which is directed toward the inlet of the main valve, is faced directly with tank pressure. The lower piston surface area is less than the upper one so that the piston is pushed down against its seat and the valve is normally closed. If the tank pressure increases and reaches a predetermined pressure, the pilot valve opens and vents the pressure space above the main piston. Thus the force acting on the main piston from below becomes comparatively greater and the main valve opens. If the tank pressure drops to the closing pressure of the pilot valve, the pilot valve closes and the tank pressure is applied to the upper piston surface so that the main valve closes as well.
According to the IMO Rule A 328 (IX) Chapter XV (XV.1), liquified gas tanks of liquified gas tankers are only allowed to be loaded up to 98% of the geometric tank volume at the opening pressure of the safety relief valve even if a reliquification plant or a device for stabilization of the pressure is provided. In the case of pressure tanks, for example full pressure or semi-pressure tankers, the maximum load filling can only be reached if immediately after loading, the saturation pressure has the same value as the opening pressure of the safety relief valve while loading the refrigerated liquified gas. If the saturation pressure immediately after loading for example is 1 bar only (e.g., butane at density=0.594) the maximum possible filling of 98% cannot be reached because according to IMO the tank must be loaded in such a way that with a pressure of 5.5 bar (e.g., butane at density-0.52) it would be filled to the maximum, i.e. 98%.
The IMO rules are intended to prevent overfilling of the tank in case of heating up of the load, for example in case of fire. The expansion caused by heat input when working normally, however, especially thermal radiation or heat conduction, is normally prevented by a reliquification plant. The restriction of the filling rate implies (according to the type of liquified gas) an unnecessary loss of up to 15%. In order to prevent this disadvantage a so-called auxiliary pilot is provided with the aforesaid safety relief valve of the AGCO Company series 95. This auxiliary pilot is temporaily applied to the primary pressure adjustment of the pilot valve. Like the primary pressure adjustment, the auxiliary pilot includes a spiral spring whose spring tension essentially determines the adjustment pressure of the pilot valve. By installing the corresponding auxiliary pilot, the pilot valve can be adapted to the physical parameters of the loading.
The solution with this known auxiliary pilot, however, is disadvantageous because it is difficult to handle. As operation of an auxiliary pilot is determined by the pressure of its spiral spring and the auxiliary is therefore only suitable for a very restricted pressure range, a graduated range of auxiliary pilots must be made available to cover the possible adjustment pressure choices between 1 bar and the admissible maximum pressure of the tank. Normally the auxiliary pilots of each tank in the tanker have to be changed when commencing loading; the new auxiliary pilots which are to be installed have to be chosen according to the saturation pressure of the specific liquified gas being loaded. As the auxiliary pilot is connected with the primary pressure adustment by several screws, special assembly works are necessary.
Various mistakes may occur when changing auxiliary pilots. An auxiliary pilot may be installed to effect operation at a wrong pressure value, due for example to a false reading of specifications. Little constituent particles, especially dirt, also may fall into the primary pressure adjustment during installation, changing its operating characteristics.